Radiological well logging using neutron collimator to reduce the effect of well fluid on the log



1964 A. D. GARRISON ETAL 3,

RADIQLOGICAL WELL LOGGING USING NEUTRON COLLIMATOR TO REDUCE THE EFFECTOF WELL FLUID ON THE LOG Original Filed June 28; 1956 United StatesPatent 3,163,761 RADIOLOGICAL WELL LOGGING USING NEU- TRON COLLIMATOR T9REDUCE THE EFFET 0F WELL FLUE ON THE [AUG Allen D. Garrison, La Jolla,Calif, and Elmer Eisner, Houston, Tex., assignors to Texaco Inc, NewYork, N.Y., a corporation of Delaware Continuation of application Ser.No. 594,583, June 28, 1956. This application Oct. 29, 1962, Ser. No.236,758 Claims. (Cl. 25083.3)

The present invention relates generally to the logging of bore holes orwells through the earth by means of a source of radioactivity and adetector. More particularly, the invention relates to improvements inneutron logging in which a source of neutrons is employed to bombard theearth formations traversed by the bore hole and scattered or slowneutrons or gamma rays induced in the formations by neutron bombardmentare measured in order to provide information as to the nature of theformations.

As is generally Well known, there are two types of socalled neutronlogging. In one method a source of neutrons and a detector of neutronsdisposed within a logging instrument or housing are passed through thebore hole. Neutrons from the source pass outwardly into the formationsaround the hole and, depending upon the nature of the formationbombarded, more or less of the neutrons are scattered within theformation, some of the neutrons passing back into the hole to strike thedetector. In the other method, a source of neutrons and a gamma raydetector disposed in a housing are passed through the bore hole and ameasurement is made of the intensity of gamma rays induced in theformations by the neutron bombardment. For purposes of brevity, thefirst of these methods will be referred to as the neutron-neutronmethod, indicating that neutronspass out into the formations and thatneutrons come back into the hole and are measured. The second methodwill be referred to as the neutron-gamma method, indicating thatneutrons pass outwardly into the formations to induce gamma raystherein, some of which gamma rays pass back into the hole to bedetected.

Oneof the most serious difliculties in neutron-neutron and neutron-gammaray logging is due to the influence of the fluids in the bore holebetween the source of neutrons and the formations and between the sourceand detector. Fluid between the formation and detector is also ofconcern. The fluid normally contained within a bore hole is oftenhydrogenous oil or water or more frequently the drilling mud used in thedrilling of the bore hole. The penetration of neutrons through thehydrogen-containing fluids of the bore hole is rather limited and as aresult substantial changes are produced on the logs when changes occurin the diameter of the well. Even in the case of a bore hole of uniformdi-' ameter, i.e., with a constant amount of fluid between the earthformation and logging instrument there is ordinarily a detrimentaleffect on the neutron log due to the presence of the fluid. For example,the hydrogen of the well fluid causes rapid slowing down of the "fastneutrons from the source, thus raising the slow neutron flux atthedetector. At the same time, the hydrogen of the well fluid causes theabsorption of the sloweddown neutrons withthe subsequent emission ofcharacteristic capture gamma rays. Thus, in the case of theneutron-neutron log where the slow neutron flux ice of the bore andseparation of source and'detector all effect the observed quantity. Themagnitude of these adverse effects on the neutron log is often as greator greater than the effect due to the formation characteristics understudy.

' Various suggestions have been made in the past for minimizing theadverse effect upon a neutron ,log due to fluid in the bore hole.

non-hydrogenous material. A method and apparatus involving thistechnique is set forth, for example, .in US. Patent 2,652,496, issuedSeptember 15, 1953, to Gerhard Herzog et al. Another patent concernedwith displacement of the hydrogen-containing fluid of a bore hole is US.Patent 2,509,908, issued May 30, 1950, to K. C. Crumrine. Both of thesepatents are concerned with proposals for eliminating adverse effects forbore hole fluid by displacement thereofwith' a non-hydrogenous, i.e.,neutron-permeable, material.

Although the aforementioned techniques for minimizing the efiects ofbore hole fluid on a neutron log are of value, nevertheless, it isconsidered desirable in many instances to provide means for furtherinhibiting the eifects on the neutron log of slow neutrons or thermalneutrons produced in the bore hole fluid by the source.

It is a general object of the invention to provide improvements in welllogging employing a neutron source in the presence of a hydrogenous wellfluid.

It is still another object of the invention to provide improved meansfor directing a'neutron source into an earth formation.

It is another object of the invention to provide improved means forovercoming the adverse effects of hydrogenous well fluid in well logginginvolvingthe use of a neutron source.

It is a further object of the invention to provide means for enhancingthe effectiveness of the neutron source in housing. Means are providedfor directing neutrons to- Ward one side of the well bore, comprisingmeans for shielding the source on the opposite side thereof. Resilientmeans are further provided for continuously urging the source toward theside of the well bore toward which the neutrons are directed. Inaccordance with a general aspect of the invention, the neutron shieldconstitutes means for thermalizing fast neutrons from the source and tfor capturing the thermalized neutrons to prevent their is the quantitymeasured or in the case of the neutrongamma log where the quantitymeasured is the: gamma flux, it will'be seen that the kind of wellfluid, diameter 1 For additional objects and advantages and for a betterunderstanding of the invention, attention is now directed to thefollowing description and accompanying drawings. The features of theinvention which are believed to be novel are particularly pointed out inthe appended claims.

. -Referring to the drawing;

FIGURE 1 is a vertical sectional elevation through a plurality ofearthformations traversed by a bore hole 7 For example, it has been pro posedto displace the bore hole fluids by means of a Containing a hydrogenousbore holefiuid within which there is suspended a neutron logginginstrument;

. FIGURE 2 is a cross-sectional view of a portion of a well logginginstrument taken through the lines 2--2 I of FIGURE 1; and

FIGURE 3 is a cross-sectional view similar to that of instrument orhousing 16 suspended therein as by means 7 of a cable 17 which may alsoprovide means for electrically coupling the instrument 16 to electricalapparatus at the surface. a For example, the cable 17 may electricallycouple the instrument 16 to a suitable amplifier 18, which, in turn,maybe coupled to a recorder 19, as shown. 'A measuring device 20 isillustrated diagrammatically and acterized in that it 'is capable ofcapturing thermalized neutrons.

In accordance with a preferred mode of carrying out the invention, thefirst section 31 of the shield 22 is formed of material having theability to backscatter, i.e., reflect fast neutrons toward theformation, as well as being able to thermalize a significant portion ofthe neutrons that are not backscatter by it. The thermal neutrons thus.produced in the first member 31 are thensubject to capture by the secondmember 32 of the neutron shield 22. Preferably, the first member 31comprises deuterium oxide, generally known asheavy water, maintainedwithin a suitable container or tank 31'. Deuterium oxide has the desiredproperty of being able to reflect and/or thermalize fast neutronsimpinging thereon. In addition to embodying the aforementioned desiredcharacteristics to a high degree, deuterium oxide may be I readilymaintained in a desired configuration by means shown in contact with thecable 17 for determining the be described below in greater detail. Thesource 21 may be'of a conventional type employed in the logging art,such as a radium-beryllium source. Typical neutron sources employed as asource of primary radiation in neutron logging such as theaforementioned radium-beryl lium source produce neutrons by bombardmentof a suitable targetwith a source of alpha rays. Sources of this typealso generally emit gamma rays along with the desired neutrons.Accordingly, above the source 21, within the housing 16, there is shownasuitable gamma ray shield.

24 which may be formed of appropriate material such as 'lead. Within'theupper end of the housing 16, remote fromthe source 21, there is shown aradiation detector of an appropriate container due to the fact that itis readily maintained in the liquid phase under logging conditions. Thesecond or outer portion 32 of the neutron shield 22 is formed ofmaterial having a comparatively high capture cross-section for thermalneutrons. Preferably, the outer portion 32 of the shield should beformed of boron or a boron compound. In a general sense, however, otherthermal neutron-capturing material may be used, such as cadmium and thelike.

As illustrated in FIGURE 2, the collimating recess 30 of the neutronshield 22 may appropriately be 'of generally fan-shaped configurationand preferably subtends a comparatively small number of radians in orderthat the effective fast neutron output of the source .21 may be directedsubstantially entirely into the formation against which the instrument16 is pressed or urged by theresilent member 27. By thus directing thefast neutrons into the formation, the likelihood of thermalization ofthe neutron beam by the well fluid 11 is minimized. Thus, the secondaryradiation observed by the detector 25 is a more effective indication ofreactions that have occurred in the formation(s)- due-to bombardment bythe fast neutrons'from the source 21..

Referring now toFIGURE 3, there is shown a further embodiment employingcertain principles of the-invention.

. In this embodiment, the shield 22 may be formed of the 25 which.mayappropriately be a proportional counter such as that referred to in theaforementioned patent to Gerhard Herzog et al., or which alternately maycomprise other known typesof detectors suitable for the detection ofdesired neutrons and/orv gamma radiation, as determined by the specifictype of log being made. The

output of the detector 25 may be coupled to suitable circuitry, such asthe preamplifier 26, for enhancing the output thereof for transmissionof the logging signal to, the surface instruments 18, 19, as by means ofthe cable 17.

The housing .16 is provided with resilientmeans' for urging one sidethereof against one side of the bore hole 10 as the instrument 16 ismoved through the well bore. This may be accomplished, as illustrated inFIGURE 1,

by the provision of a resilient pressure bar 27lafiixed tothe upper andlower ends of the instrument 16 and he mg of bowed configuration suchthat it is adapted to con- ...boronto capture thethermal neutrons 850times as fast tact the well bore in a region generally opposite thecenter of the instrument. I I

Theneutron shield 22 generally surrounds the source neutrons ernmittedthereby selectively toward the side of the formation against which theinstrument 16 is main- I V 21, except for a collimating aperture 30 fordirecting tained'by thepressure member 27. The shield 22 is comprisedofa first or inner shield member 31 formed of material that'is capable ofeffective thernializing fast, neutrons; The first member 31 is, in turn,generally en closed or surrounded by a second or outer member 32,

shield 24, as shown in, FIGURE 1. It will thus be ap- I except for theside thereof toward which'the collimating aperture 30 is direct. Thesecond member 32 is charsame general configuration as that shown inFIGURE 2.

The collimating aperture 30" of the shield illustrated in FIGURE 3 maybe generally like that of FIGURE 2. However, instead of two separatesections, the shield 22 is formed of a generally uniform composition ofmaterials capable of thermalizing and capturing fast neutronsimpingingthereon from the source 21. Preferably, the

material of the shield in this embodiment comprises both boron andhydrogen. Solid ammonium borate is particularly effective as theshielding material. Ammonium borate (NH .H.B O .3H O) has a densityof'about 2.6, and contains about 11% more hydrogen in one liter than 7water. This material will thermalize fast neutrons at a -ratewhichexceeds that of pure water, and is several.

times the rate of thermalization in earth formations, or drillingfluids. However, this compound contains enough as its own hydrogen.

By the use of such a shield, a large fraction of the neutrons whichwould otherwise enter the drilling fluid ticle which cannot escape tothe detector. Although weak, i.e., soft gamma rays, are produced, theymay be readily shielded from the detector by means of a conventionalsource to counter shield such as the gamma ray I parent that thisembodiment' of the inventionafiords an improved contrastin the loggingsignalg Additional stable materials suitable for the shielding materialand containing both boron and hydrogen are as follows:

As illustrated in FIGURE 1, the collimating aperture 30 of the variousembodiments of the invention may advantageously be formed with agenerally horizontal lower boundary and generally tapered upperboundary, such that the aperture 33 widens as it proceeds outwardly fromthe source 21 in such manner as to direct neutrons outwardly andsomewhat upwardly, whereby the fast neutrons may be directed into theformations at such a location that interactions produced therein mayreadily be observed by the detector 25.

In accordance with a still further aspect of the invention, it iscontemplated that the outer shielding member 32 of the shield 22 asshown in FIGURES 1 and 2 may comprise a substantially uniformcomposition of boron and hydrogen, such as those materials specified foruse as the shielding material in the embodiment of the inventionillustrated in FIGURE 3. Preferably, in accordance with this furtheraspect of'the invention, the outer shielding member 32 should be formedof ammonium borate.

As will be appreciated by those skilled in the art, the improved neutronshield construction set forth in the preceding paragraph aiiords theadvantage of providing a neutron shield wherein the inner shieldingmember 31 is effective in both reflecting and thermalizing neutrons andwherein the outer shielding member 32 is effective in both thermalizingneutrons and capturing thermal neutrons. This combination of shieldingmaterials provides a novel and efiective neutron shield for use inneutron logging, as discussed above.

This application is a continuation of our application Serial No.594,583, filed June 28, 1956 and now abandoned.

While specific embodiments have been shown and described, it will beunderstood that various modifications may be made without departing fromthe principles of the invention. The appended claims are, therefore,intended to cover any modifications within the true spirit and scope ofthe invention.

We claim:

1. A device for logging a bore hole traversing subsurface earthformations and containing a hydrogenous well fluid comprising aninstrument housing adapted to be passed through the bore hole, means forsupporting said housing in the bore hole, a source of neutrons and adetector of secondary radiations positioned in spaced relation along thelongitudinal axis of the instrument, resilient means for continuouslyurging the housing toward one side of the well bore and means fordirecting neutrons outwardly from the source toward the formations onsaid side of the well bore, said directing means comprising a neutronshield generally enclosing said source and having a collimating aperturefacing said one side, said shield including'an inner shielding membercomprising deuterium oxide characterized as both a moderator andreflector of neutrons whereby said inner shielding member is adapted andarranged to thermalize and reflect neutrons toward said one side and anouter shielding member comprising boron and hydrogen in substantiallyuniform distribution generally enclosing the side of said inner memberremote from said aperture, said outer member being characterized as botha moderator and absorber of neutrons. whereby said outer mem ber isadapted and arranged to thermalize neutrons and to capture thermalneutrons, whereby neutrons are directed outwardly from the source intothe formation to the substantial exclusion or" the well 'fiuid.

2. A device as in claim 1 wherein the material of said outer shieldingmember'comprises ammonium bo'rate.

3. A device for logging a bore hole traversing subsurface earthformations and containing a hydrogenous well fluid comprising aninstrument housing adapted to be passed through the bore hole, means forsupporting said housing in the bore hole, a source of neutrons and adetector of secondary radiations positioned in spaced relation along thelongitudinal axis of the instrument, resilient means for continuouslyurging the housing toward one side of the well bore and means fordirecting neutrons outwardly from the source toward the formations onsaid side of the well bore, said directing means comprising a neutronshield generally enclosing said source and having a collimating aperturefacing said one side, said shield including an inner shielding membercharacterized as both a moderator and reflector of neutrons havingsignificant reflective properties for neutrons and a low absorptioncross-section for neutrons whereby said inner shielding member isadapted and arranged to thermalize and reflect neutrons toward said oneside and an outer shielding member generally enclosing the side of saidinner member remote from said aperture, said outer member beingcharacterized as both a moderator and absorber of neutrons whereby saidouter member is adapted and arranged to thermalize neutrons and tocapture thermal neutrons, whereby neutrons are directed outwardly from.the source into the formation to the substantial exclusion of the wellfluid.

4. A device as in claim 3 wherein the material of said outer shieldingmember comprises ammonium borate.

5. A device as in claim 3 wherein said outer shielding member comprisesboron and hydrogen in substantially uniform distribution.

OTHER REFERENCES Fermi: U.S.A.E.C.,Report NP 2385 (1951). lation of Ric.Scient. VII-2, 134936.

Trans-

1. A DEVICE FOR LOGGING A BORE HOLE TRAVERSING SUBSURFACE EARTHFORMATIONS AND CONTAINING A HYDROGENOUS WELL FLUID COMPRISING ANINSTRUMENT HOUSING ADAPTED TO BE PASSED THROUGH THE BORE HOLE, MEANS FORSUPPORTING SAID HOUSING IN THE BORE HOLE, A SOURCE OF NEUTRONS AND ADETECTOR OF SECONDARY RADIATIONS POSITIONED IN SPACED RELATION ALONG THELONGITUDINAL AXIS OF THE INSTRUMENT, RESILIENT MEANS FOR CONTINUOUSLYURGING THE HOUSING TOWARD ONE SIDE OF THE WELL BORE AND MEANS FORDIRECTING NEUTRONS OUTWARDLY FROM THE SOURCE TOWARD THE FORMATIONS ONSAID SIDE OF THE WELL BORE, SAID DIRECTING MEANS COMPRISING A NEUTRONSHIELD GENERALLY ENCLOSING SAID SOURCE AND HAVING A COLLIMATING APERTUREFACING SAID ONE SIDE, SAID SHIELD INCLUDING AND INNER SHIELDING MEMBERCOMPRISING DEUTERIUM OXIDE CHARACTERIZED AS BOTH A MODERATOR ANDREFLECTOR OF NEUTRONS WHEREBY SAID INNER SHIELDING MEMBER IS ADAPTED ANDARRANGED TO THERMALIZE AND REFLECT NEUTRONS TOWARD SAID ONE SIDE AND ANOUTER SHIELDING MEMBER COMPRISING BORON AND HYDROGEN IN SUBSTANTIALLYUNIFORM DISTRIBUTION GENERALLY ENCLOSING THE SIDE OF SAID INNER MEMBERREMOTE FROM SAID APERTURE, SAID OUTER MEMBER BEING CHARACTERIZED AS BOTHA MODERATOR AND ABSORBER OF NEUTRONS WHEREBY SAID OUTER MEMBER ISADAPTED AND ARRANGED TO THERMALIZE NEUTRONS AND TO CAPTURE THERMALNEUTRONS, WHEREBY NEUTRONS ARE DIRECTED OUTWARDLY FROM THE SOURCE INTOTHE FORMATION TO THE SUBSTANTIAL EXCLUSION OF THE WELL FLUID.